Until June of 1791, the National Assembly tried to build a constitutional monarchy, even as it faced increasing hostility among the great powers of Europe, all of which were monarchies, along with problems with inflation and hunger in the countryside. In June of 1791, the king and his family fled Paris, but were caught on the border (supposedly by a postal worker who recognized the king from his portrait on coins). It was soon discovered that the royal family had been corresponding with foreign monarchs and nobles, hoping to inspire an invasion from abroad to restore the king to the throne and to end the Revolution by force. The situation rapidly radicalized as the prestige of the king was destroyed overnight; even as the new French Constitution was formally passed in October of 1791, making France a constitutional monarchy, the king himself was under house arrest.
The latter situation prompted the kings of Austria and Prussia to call upon the monarchs of Europe to fully restore Louis XVI to control of his country, although they did not yet declare war on France. Radical elements of the National Assembly, however, anticipated war and convinced the Assembly to declare a preemptive war on Austria in April of 1792; Prussia soon joined in an alliance with Austria against France. The Assembly dispatched the new National Guard and a hastily-assembled army, many of whom were former soldiers of the royal army, against the forces of Austria and Prussia along the French border.
In September of 1792, as the war began in earnest and the king languished in prison, a new constitution was instituted that formally abolished the monarchy and made France into a republic with universal manhood suffrage. This was the first time in the history of Europe that every adult male was allowed the right to vote regardless of wealth or status. In just over three years, France had gone from an absolute monarchy to the first major experiment in democracy since the days of the Roman Republic nearly two thousand years earlier.
In January of 1793, Louis XVI was executed as a traitor to the republic after heated debate and a close vote in the Assembly. The war grew as Britain and the Dutch Republic joined with Prussia and Austria against France, further increasing the military pressure on the French borders. The middle part of 1793 saw fear of foreign invasion and food shortages, along with royalist uprisings in parts of France itself. The result was the appointment of a dictatorial emergency committee, the Committee for Public Safety, headed by twelve of the most radical members of the republican government.
The twelve members of this committee would rule France from September 1793 to July 1794 as a dictatorial council, charged with defending the Revolution from both its external enemies and internal rebels. It was extremely successful in the former regard, issuing a levée en masse, or total mobilization for war, which swelled the ranks of the French forces and held the Austrian and Prussian armies in check. Meanwhile, the Revolutionary government set up a subsistence committee to develop and elaborate a system of price controls, requisitions, and currency regulation, backed by police power. The committee restored order to rebellious areas by sending its members on missions with instructions for ruthless repression, again backed by violence.
Thus, just five years after the Revolution had begun, control was now in the hands of a small dictatorial committee of radicals who used violent repression to hold the nation together, continue the war against almost all of Europe, and soon, to pass even more radical measures. They made extensive use of the guillotine, a new “humane” technology of execution named after the medical doctor who invented it, and their leader was the (in)famous Maximilien Robespierre, whom his followers called "the Incorruptible" for his single-minded focus on seeing the Revolution succeed.
Under Robespierre's leadership, the Committee for Public Safety attempted to reorganize and "rationalize" French society as a whole, not just win wars. The Revolutionary government passed a number of radical measures under Robespierre’s leadership. First, it sponsored the creation of the metric system. From a unsystematic smattering of different standards of weights and measures across France, the Revolutionary government oversaw the invention and use of a simple, unified system based on increments of ten (i.e. 100 centimeters is equal to 1 meter, 1,000 meters is equal to one kilometer, 1,000 grams is equal to 1 kilogram, etc.). Of all the changes instituted by the Revolutionary government during its radical phase, this was to be the most successful and long-lasting.
Since the members of the committee believed that not just France, but the world was on the threshold of a new era, they proclaimed the creation of a new calendar that began on September 22, 1792 (Day 1, Year 1), the day that the republic had been declared. All of history was to follow from that first day. Likewise, new ten-day weeks were introduced, with new four-week months named after their weather rather than arbitrary historical figures (e.g. the month of August, named after Augustus Caesar, was renamed "Thermidor," which means "hot." February became "Brumaire," which means "foggy," and April became "Prairial," meaning "springlike.") Year-end celebrations were planned to pay tribute to the Revolution itself in quasi-religious ceremonies presided over by republican officials.
In perhaps the most astonishing campaign, the Revolutionary state launched a major attempt to “de-Christianize” the nation, removing crosses from buildings and graveyards and renaming churches “temples to reason.” The cathedral of Notre Dame in the center of Paris was stripped of its Christian iconography, and Robespierre oversaw new ceremonies meant to worship a (newly invented) supreme being of reason. This was the culmination of the anticlerical measures that had begun in first year of the Revolution, with the seizure of church lands and property, but it now aimed at nothing less than the suspension of Christianity itself in France. In something of a symbolic parallel, the committee also had the bodies of dead French kings disinterred and dumped into a common grave (the corpse of Louis XIV landed on that of his grandfather, Henry IV).
To enforce its will and ensure “security,” the Committee for Public Safety oversaw what was later dubbed "The Terror," as suspected traitors were arrested, interrogated, and confronted with the possibility of imprisonment or execution. While estimates vary considerably, somewhere between 35,000 - 55,000 accused enemies of the Revolution were executed or died in prison during the Terror, which was further intensified by widespread imprisonment (totaling half a million people, 3% of the adult population). To impose its policies on grain procurement and prices, the government had to rely largely on local organizations of militants who often terrorized the very peasants they were supposed to represent. Likewise, the most significant battles fought by French troops were against royalist (French) rebels, not foreign soldiers.
In fact, the bloodiest repression seen during the Terror happened far from Paris, and did not involve any guillotines. A western region of France, the Vendée, had been the site of the largest royalist insurrection against the Revolution in early 1793, featuring a rebel army of conservative peasants. It took until the summer for the royalists to be defeated, and in the aftermath of that defeat the revolutionary army inflicted a form of revenge against the people of the region that came close to outright genocide. Men and women were slaughtered regardless of whether or not they had participated in the uprising, villages were burned to the ground, and the death toll easily exceeded 100,000 people (some estimates place the number far higher).
Against the backdrop of the Terror, many members of the Revolutionary government itself began to fear for their lives. Likewise, the mandate for the committee’s very existence - protecting the Revolution against its foreign and domestic enemies - was made somewhat obsolete when French forces won major victories against Prussia and Austria in the summer of 1794. Robespierre inspired revulsion and fear among even some of his erstwhile supporters because of his fanatical devotion to the Revolutionary cause and his overt attachment to using terror to achieve his ends. Thus, in July of 1794 a conspiracy of worried Revolutionaries succeeded in arresting, briefly trying, and then executing Robespierre as a tyrant. The Committee of Public Safety was dissolved.
After the fall of Robespierre the Revolution began to slide away from its most radical positions. A government of property owners took over under a new “Directory” in 1795, which rescinded price controls and ended the abortive attempt to de-Christianize the nation. A wave of reprisals against former radicals known as the “white terror” saw tens of thousands murdered (as many died in the white terror as had under the Committee of Public Safety’s campaigns of persecution). France remained at war with most of the rest of the Europe, even as royalist uprisings continued in areas across the nation itself. It was in this context of violence and insecurity that, In October of 1795, a young, accomplished general named Napoleon Bonaparte put down a royalist insurrection in Paris and came to the attention of ambitious politicians within the Directory.